Flat panel display device and method for fabricating same

ABSTRACT

Provided is a flat panel display device and a method for fabricating the same. The flat panel display device comprises a first substrate, a light emitting unit, a second substrate, and insulating films. The light emitting unit comprises thin film transistors positioned on the first substrate, a first electrode electrically connecting with the thin film transistors, a second electrode facing the first electrode, and an emission layer or a liquid crystal layer interposed between the first and second electrodes. The second substrate is sealed with the first substrate by an ultraviolet curing sealant, and has a greater thermal expansion coefficient than the first substrate. The insulating films are positioned on one or more surfaces of the first and/or second substrates.

This application is a divisional of U.S. patent application Ser. No.11/639,698 filed on Dec. 14, 2006, which claims the benefit of KoreaPatent Application No. 10-2006-0044353, filed on May 17, 2006, theentire contents of which is incorporated herein by reference for allpurposes as if fully set forth herein.

BACKGROUND

1. Field of the Invention

The present invention relates to a flat panel display device and amethod for fabricating the same.

2. Discussion of the Related Art

In recent years, flat panel displays (FPD) are increasing in importanceas multimedia is developed. In response to this, several flat typedisplays such as liquid crystal display (LCD), plasma display panel(PDP), field emission display (FED), and organic light emitting device(OLED) are being put to practical use. Among them, the liquid crystaldisplay has an excellent visibility, and has lower average consumptionpower and heat release rate compared with a cathode ray tube. The fieldemission display is attracting attention as a next generation flat paneldisplay device because it has a great response speed of 1 ms or less,has low power consumption, and has no viewing angle problems because ofits self-emission.

Methods for driving flat panel display devices are classified into apassive matrix method, and an active matrix method using a thin filmtransistor (TFT). The passive matrix method is a method in which ananode and a cathode are formed to cross at right angles, and a line isselected, thereby driving the flat panel display device. In comparisonwith this, the active matrix method is a method in which the thin filmtransistor connects to each pixel electrode of indium tin oxide (ITO),and the flat panel display device is driven depending on a voltagesustained by the capacitance of a capacitor connecting to a gateelectrode of the thin film transistor.

FIG. 1 is a cross-sectional view illustrating a conventional flat paneldisplay device. Referring to FIG. 1, the flat panel display devicecomprises a first substrate 100, a light emitting unit 110, and a secondsubstrate 120 opposing the first substrate 100. The first substrate 100and the second substrate 120 are sealed by a sealant 130, therebysealing the light emitting unit 110. The light emitting unit 110 maycomprise a first electrode, a second electrode, and an emission layer ora liquid crystal layer interposed between the first and secondelectrodes. The flat panel display device based on the active matrixmethod may further comprise a thin film transistor electricallyconnecting with the first electrode.

A substrate used for the flat panel display device may be made of glass,plastic, or metal. In general, the substrate formed of glass istypically used. The glass may be a non-alkali glass, a soda lime glass,or a borosilicate glass. The non-alkali glass contains less than 0.1 wt% of Na₂O, the borosilicate glass contains 0.1 wt % to 1 wt % of Na₂O,and the soda lime glass contains more than 1 wt % of Na₂O. The soda limeglass is also called an alkali glass.

In the flat panel display device comprising the thin film transistorbased on the active matrix method, non-alkali glass is typicallyemployed as the first substrate. This is to protect the thin filmtransistor from alkali ions diffused from the substrate in a process offabricating the thin film transistor. In other words, when the alkaliions are diffused into a channel region of a semiconductor layer, thealkali ions change a semiconductive property of the channel region intoa conductive property. This deteriorates an off characteristic of thethin film transistor, increases a leakage current, and causes a residualimage while driving the display. Accordingly, it is desirable to employnon-alkali glass in the first substrate in a flat panel display deviceusing the active matrix method, in order to solve the above problems.However, the non-alkali glass substrate is more expensive than otherglass substrates and thus, may increase a price of the flat paneldisplay device. In order to reduce a manufacture cost, the firstsubstrate having the thin film transistor may employ the non-alkaliglass, and the second substrate sealed with the first substrate mayemploy the soda lime glass.

The first substrate comprising the thin film transistor and the lightemitting unit is sealed with the second substrate by the sealant,thereby sealing the light emitting unit formed on the first substrate.In order to cure the sealant, the sealant is subject to irradiation ofultraviolet rays and then, is heat-treated for about one hour at atemperature of about 230° C.

The heat treatment causes a thermal expansion of the sealed firstsubstrate and second substrate. Because the first substrate and thesecond substrate are of materials different from each other, there is adrawback in that the sealed first substrate and second substrate arebent in one direction as shown in FIG. 1. In other words, a differencebetween thermal expansion coefficients of two substrates may cause thebending of the flat panel display device. This causes a deterioration ofyield and a reduction of reliability.

BRIEF SUMMARY

Accordingly, the present invention is to provide a flat panel displaydevice and a method for fabricating the same, for preventing bending ina sealing process, and improving yield and reliability.

In one aspect, there is provided a flat panel display device. The flatpanel display device comprises a first substrate, a light emitting unit,a second substrate, and insulating films. The light emitting unitcomprises thin film transistors positioned on the first substrate, afirst electrode electrically connecting with the thin film transistors,a second electrode facing the first electrode, and an emission layer ora liquid crystal layer interposed between the first and secondelectrodes. The second substrate has a greater thermal expansioncoefficient than the first substrate and is sealed with the firstsubstrate by an ultraviolet curing sealant. The insulating films arepositioned on one or more surfaces of the first and/or secondsubstrates.

In another aspect, there is provided a method for fabricating a flatpanel display device. The method comprises preparing a first substrate,and a second substrate having a greater thermal expansion coefficientthan the first substrate. An insulating film is formed on one or moresurfaces of any one of the first and second substrates. A light emittingunit is formed on the first substrate. The light emitting unit comprisesthin film transistors, a first electrode connecting with the thin filmtransistors, a second electrode facing the first electrode, and anemission layer or a liquid crystal layer interposed between the firstand second electrodes. The first and second substrates are sealed usinga sealant.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like numerals refer to like elements.

FIG. 1 is a cross-sectional view illustrating a conventional flat paneldisplay device;

FIG. 2 is a cross-sectional view illustrating a flat panel displaydevice according to an embodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a pixel structure of aflat panel display device according to an embodiment of the presentinvention;

FIG. 4 is a cross-sectional view illustrating a flat panel displaydevice according to another embodiment of the present invention; and

FIG. 5 is a cross-sectional view illustrating a flat panel displaydevice according to a further another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PRESENTLY PREFERREDEMBODIMENTS

Embodiments of the present invention will be described in a moredetailed manner with reference to the drawings. However, the describedembodiments may be corrected in a different way, all without departingfrom the spirit or scope of the present invention. In the drawings, whenany layer is formed “on” another layer or substrate, it means that anylayer may be directly formed on another layer or substrate, or a thirdlayer may be interposed therebetween. Like reference numerals designatelike elements throughout the specification.

FIG. 2 is a cross-sectional view illustrating a flat panel displaydevice according to an embodiment of the present invention. FIG. 3 is across-sectional view illustrating a pixel structure of a flat paneldisplay device according to an embodiment of the present invention.

Referring to FIG. 2, the flat panel display device according to anembodiment of the present invention comprises a first substrate 200, anda second substrate 220 facing the first substrate 200 and having agreater thermal expansion coefficient than the first substrate 200. Thefirst substrate 200 may be of non-alkali glass in order to prevent thinfilm transistors to be subsequently formed on the first substrate 200from being contaminated by alkali ions. The second substrate 220 may bea soda lime glass substrate or a borosilicate glass substrate. Thenon-alkali glass has a thermal expansion coefficient of about 38×10⁻⁷/°C., and the soda lime glass has a thermal expansion coefficient of about90×10⁻⁷/° C. A first insulating film 205 is positioned on an outersurface of the first substrate 200. A second insulating film 225 ispositioned on an outer surface of the second substrate 220. The firstand second insulating films 205 and 225 may have a thermal expansioncoefficient of 0 or less. In other words, the first and secondinsulating films 205 and 225 may comprise material having a thermalexpansion coefficient of 0 or less, such as aluminum oxide (Al₂O₃),yttrium oxide (Y₂O₃), and silicon nitride (Si₃N₄). The first and secondinsulating films 205 and 225 may be formed at a thickness of about 50 Åto 5000 Å using the well-known process such as plasma enhanced chemicalvapor deposition (PECVD) or low-pressure chemical vapor deposition(LPCVD).

In contrast, the first insulating film 205 may have a greater thermalexpansion coefficient than the first substrate 200, and the secondinsulating film 225 may have a less thermal expansion coefficient thanthe second substrate 220.

The first insulating film 205 or the second insulating film 225 may beof transparent material depending on the direction in which light isemitted from the flat panel display device. In other words, in atop-emission flat panel display device, the second insulating film 225should be of transparent material.

A light emitting unit 210 is positioned on the first substrate 200comprising the first insulating film 205. The light emitting unit 210comprises a plurality of pixels. Each pixel may comprise a thin filmtransistor, a first electrode, a second electrode, and an emission layeror a liquid crystal layer interposed between the first and secondelectrodes. The pixel structure of the flat panel display deviceaccording to an embodiment of the present invention will be describedbelow with reference to FIG. 3.

Referring to FIG. 3, a buffer layer 305 is positioned on the firstsubstrate 300. The thin film transistor comprises a semiconductor layer310, a gate insulating layer 320, a gate electrode 330, and source/drainelectrodes 350 a and 350 b, and is positioned on the buffer layer 305.The semiconductor layer 310 may be of amorphous silicon orpolycrystalline silicon. Impurity ions may be implanted into thesemiconductor layer 310, thereby forming source, drain, and channelregions in the semiconductor layer 310. The gate electrode 330 ispositioned on the gate insulating layer 320 so that it corresponds to apredetermined region of the semiconductor layer 310. The gate electrode330 and the source/drain electrodes 350 a and 350 b are insulated witheach other using an inter-insulating layer 340. The source/drainelectrodes 350 a and 350 b electrically are connected to portions of thesemiconductor layer 310 through a first contact hole 335 a and a secondcontact hole 335 b positioned in the inter-insulating layer 340 and thegate insulating layer 320. A passivation layer 360 is positioned on theabove constructed thin film transistor. A via hole 365 is positioned inthe passivation layer 360, and exposes the drain electrode 350 b of thethin film transistor. The first electrode 370 is positioned on thepassivation layer 360, and connects with the drain electrode 350 b ofthe thin film transistor through the via hole 365. A pixel defining film380 is positioned on the first electrode 370, and comprises an opening385 for exposing a part of the first electrode 370. The emission layer390 is of organic matter, and is positioned in the opening 385. Thesecond electrode 395 is positioned on the pixel defining film 380comprising the emission layer 390.

In an embodiment of the present invention as described, the emissionlayer is interposed between the first electrode and the secondelectrode, but the liquid crystal layer may be also interposed betweenthe first electrode and the second electrode.

Referring again to FIG. 2, the first substrate 200 comprising the firstinsulating film 205 and the light emitting unit 210 and the secondsubstrate 220 comprising the second insulating film 225 are sealed usinga sealant 230, thereby sealing the light emitting unit 210. The sealant230 may be an ultraviolet curing sealant. The sealant 230 may be coatedand then, may be cured using a heat-treatment for one hour or more at ahigh temperature of 200° C. or more.

As discussed above, in a conventional flat panel display device, aphenomenon occurs in which the flat panel display device is bent in aheat-treatment process due to the difference between the thermalexpansion coefficients of the first substrate 200 and the secondsubstrate 220. However, in the flat panel display device according to anembodiment of the present invention, the first insulating film 205 andthe second insulating film 225 are formed on one or more surfaces of thefirst substrate 200 and the second substrate 220 to prevent the bendingphenomenon.

If the first and second insulating films 205 and 225 are of materialhaving the thermal expansion coefficient of 0 or less, they are notthermally expanded even though the flat panel display device is under ahigh temperature for a long time in a sealing process using the sealant.Thus, a phenomenon in which the first and second substrates 200 and 220are bent due to the difference between the thermal expansioncoefficients may be suppressed, thereby preventing the bending of theflat panel display device.

The first insulating film 205 comprises material having a greaterthermal expansion coefficient than the first substrate 200, and thesecond insulating film 225 comprises material having a less thermalexpansion coefficient than the second substrate 220. If so, thedifference between the thermal expansion coefficients of the first andsecond substrates 200 and 220 may be compensated. Accordingly, thebending of the flat panel display device may be prevented in theheat-treatment process.

Table 1 below shows thermal expansion coefficients of materials that maybe used as the insulating films.

TABLE 1 Material Thermal expansion coefficient Si₃N₄ 0 (up to 1200° C.)Y₂O₃ 0 (up to 1000° C.) SiO₂ 5 × 10⁻⁷/° C. Ge₂O 77 × 10⁻⁷/° C.  B₂O₃ 150× 10⁻⁷/° C.  Al₂O₃ 6 × 10⁻⁷/° C.

In the described embodiment of the present invention the first andsecond insulating films 205 and 225 are positioned on the outer surfacesof the first and second substrates 200 and 220, but they may be eitherpositioned on inner surfaces or both surfaces of the first and secondsubstrates 200 and 220, without departing from the sprit and scope ofthe present invention.

FIG. 4 is a cross-sectional view illustrating a flat panel displaydevice according to another embodiment of the present invention.Referring to FIG. 4, the flat panel display device according to anotherembodiment of the present invention comprises a first substrate 400, anda second substrate 420 facing the first substrate 400 and having agreater thermal expansion coefficient than the first substrate 400. Thefirst substrate 400 may be a non-alkali glass substrate, and the secondsubstrate 420 may be a soda lime glass substrate or a borosilicate glasssubstrate.

A second insulating film 425 is positioned on an outer surface of thesecond substrate 420. A thermal expansion coefficient of the secondinsulating film 425 may be smaller than a thermal expansion coefficientof the second substrate 420.

A light emitting unit 410 is positioned on the first substrate 400. Thelight emitting unit 410 is comprised of a plurality of pixels. Eachpixel may comprise at least one thin film transistor, a first electrodeconnecting with the thin film transistor, a second electrode facing thefirst electrode, and an emission layer or a liquid crystal layerinterposed between the first and second electrodes.

The first substrate 400 comprising the light emitting unit 410, and thesecond substrate 420 comprising the insulating film 425 are sealed usinga sealant 430, thereby sealing the light emitting unit 410. The sealant430 may be an ultraviolet curing sealant. A heat-treatment process maybe performed in a high temperature for a long time to cure the sealant430.

In the flat panel display device according to another embodiment of thepresent invention, the second insulating film 425 having a less thermalexpansion coefficient than the second substrate 420 is positioned on thesecond substrate 420 having a greater thermal expansion coefficient thanthe first substrate 400. Thus, the second insulating film 425 may causea reduction of the difference between the thermal expansion coefficientsof the first substrate 400 and the second substrate 420. Therefore, aphenomenon in which the flat panel display device is bent in theheat-treatment process may be prevented.

FIG. 5 is a cross-sectional view illustrating a flat panel displaydevice according to a further another embodiment of the presentinvention. Referring to FIG. 5, the flat panel display device accordingto a further another embodiment of the present invention comprises afirst substrate 500, a light emitting unit 510 positioned on the firstsubstrate 500, and a second substrate 520 that is sealed with the firstsubstrate 500 by a sealant 530 to seal the light emitting unit 510 andcomprises an insulating film 525 on its inner surface.

The flat panel display device according to a further another embodimentof the present invention is different only in the position of itsinsulating film, from the flat panel display device according to anotherembodiment of the present invention. In other words, the secondinsulating film 525 having the less thermal expansion coefficient thanthe second substrate 520 is formed on the inner surface of the secondsubstrate 520 to reduce the difference between the thermal expansioncoefficients of the first substrate 500 and the second substrate 520.

In FIGS. 4 and 5, it is shown that the insulating film having the lessthermal expansion coefficient than the second substrate is positioned onany one surface of the second substrate, but it is not intended to limitthe present invention. In other words, the insulating film having theless thermal expansion coefficient than the second substrate may bepositioned on both surfaces of the second substrate. Alternately, theinsulating film having the greater thermal expansion coefficient thanthe first substrate may be positioned on any one surface of the firstsubstrate.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A method for fabricating a flat panel display device, the methodcomprising: preparing a first substrate, and a second substrate having agreater thermal expansion coefficient than the first substrate; formingan insulating film on one or more surfaces of any one of the firstand/or second substrates; forming a light emitting unit on the firstsubstrate, the light emitting unit comprising thin film transistors, afirst electrode connecting with the thin film transistors, a secondelectrode opposing the first electrode, and an emission layer or aliquid crystal layer interposed between the first and second electrodes;and sealing the first and second substrates using a sealant.
 2. Themethod of claim 1, wherein the insulating film is of material having agreater thermal expansion coefficient than the first substrate on anyone or more surfaces of the first substrate.
 3. The method of claim 1,wherein the insulating film comprises material having a less thermalexpansion coefficient than the second substrate on any one or moresurfaces of the second substrate.
 4. The method of claim 1, wherein theforming of the insulating film comprising: forming a first insulatingfilm on one or more surfaces of the first substrate; and forming asecond insulating film on one or more surfaces of the second substrate.5. The method of claim 4, wherein the first insulating film and thesecond insulating film comprise material having a thermal expansioncoefficient of 0 or less.
 6. The method of claim 4, wherein the firstinsulating film comprises material having a greater thermal expansioncoefficient than the first substrate, and the second insulating film isof material having a less thermal expansion coefficient than the secondsubstrate.